Stair-step thermal recovery of oil

ABSTRACT

A method for producing a viscous oil from a reservoir having a number of permeable oil saturated layers separated by impermeable barriers through which heat may be conducted. According to the method, a water zone and a zone of saturation transition between a water zone and an overlying oil zone of a first formation are used as conduits for carrying heat into the formation. This heat is employed to reduce the viscosity of oil in the formation thus improving the injectivity of the formation and facilitating the initiation of a steam flood therein. Hear from this stream flood is conducted, through an adjoining impermeable barrier and into an adjacent formation which is then steam flooded.

United States Patent Breithaupt et al.

[54] STAIR-STEP THERMAL RECOVERY OF OIL [72] Inventors: John T.Breithaupt, Houston, Tex.

77025 ;'Raymond T. Garcia, Houston, Tex. 77027 [73]- Assignee: Shell OilCompany, New York, I

[22] Filed: July 14, 1970 21 Appl. No.: 54,721

[52] US. Cl. ..166/252, 166/269, 166/272 [51] Int. Cl. ..E2lb 43/24 [58]Field ofSearch ..166/269, 272, 274, 252

[56] References Cited UNITED STATES PATENTS 3,167,120 l/1965 Pryor..166/272 3,193,008 7/1965 Moore ..166/258 3,439,742 4/1969 Durie..166/272 3,180,413 4/1965 Willman ..166/272 3,372,750 3/1968 Satter etal. 166/272 3,500,917 3/1970 Lehner et al. ..166/272 3,537,526 11/1970Offeringa ..166/272 X OTHER PUBLICATIONS k; tr um... 29411550 1 Haab 1]1 1 Sept. 19, 1972 Reservoir Engineering, McGraw-Hill Book Co., 1962,pp. 21-1; 22-1.

Uren; Petroleum Production Engineering, Oil Field Exploitation, 3rd Ed.McGraw-Hill Book Co., N.Y., 1953,pp. l2, 30,31,38-41.

Primary Examiner-Stephen .I. Novosad Attorney-Harold L. Denkler andTheodore E. Bieber [571 V ABSTRACT A method for producing a viscous oilfrom a reservoir having a number of permeable oil saturated layersseparated by impermeable barriers through which heat may be conducted.According to the method, a water zone and a zone of saturationtransition between a water zone and an overlying oil zone of a firstformation are used as conduits for carrying heat into the for- 2 Claims,2 Dravving Figures PATENTEDSEPIQISTE 3 692 111 INVENTORS:

J.T. BREITHAUPT .R.T. GA IA BY- THE? ATTORNEY BACKGROUND OF THEINVENTION 1. Field of the Invention This invention relates to a methodfor producing hydrocarbons from a subsurface reservoir. Moreparticularly, this invention relates to a method for treating anoil-bearing reservoir with steam and/or hot water to increase therecovery of hydrocarbons therefrom.

2. Description of the Prior Art In many areas of the world, reservoirsof low A.P.I. gravity crude oil exist which are difficult to producebecause the high viscosity of such low gravity oils makes themsubstantially immobile within the reservoir. It is well known that theviscosity of most crude petroleum is temperature dependent and that theviscosity of the oil in a given reservoir may be decreased by a factoroften on the order of 50 to 1 ,000 times by an increase in thetemperature of that oil above the reservoir temperature on the order of100 F. To this end, a number of methods for heating oil in a petroleumreservoir have been successfully employed.

. Among these is the injection of steam and/or hot water into thereservoir to heat the oil, thereby lowering its viscosity, and to drivethe heated oil to a producing well.

Some oil reservoirs which contain viscous crudes are composed of anumber of layers or formations of sandstone or other permeableoil-bearing rocks separated by relatively thin impermeable barriersthrough which heat may be conducted such as layers of shale. It has beensuggested that heat may be provided by conduction to one of two adjacentoil-bearing layers of such a reservoir by carrying out a conventionalsteam flood in the other of the adjacent layers. However, in somelayered reservoirs, the oil in each of the oil-bearing layers may be soviscous and immobile at the naturally occurring reservoir temperaturethat it is difficult or impossible to initiate a steam and/or hot waterflood in any of the layers by injecting a hot aqueous fluid solely intoa predominately oil-saturated zone.

It has also been suggested that an oil-bearing layer may be warmed byheat conducted from a steam flood of an adjacent water sand. However, insome cases it may be economically impractical to heat an adjacentoil-bearing layer by steam flooding a water-saturated layer because itmay take a substantial preheat period before significant amounts of oilare produced from the adjacent oil zone.

SUMMARY OF THE INVENTION It is not uncommon for at least one oil-bearingsand in a reservoir having a number of oil-bearing layers separated byimpermeable barriers to include at least one oil-bearing layer which hasa predominately watersaturated interval or zone near the bottom of thatlayer.

The present invention provides a method for producing oil from a viscousoil containing reservoir having a number of oil-bearing layers orformations separated by impermeable barriers through which heat may beconducted and having at least one layer or formation (the first treatedformation") which includes an upper zone predominately saturated withviscous oil, a lower zone predominately saturated with water, and a zoneof saturation transition between the upper and lower zones in which oilsaturation decreases and water saturation increases with increasingdepth. According to the present invention, the transition zone and thewater zone of the oil-bearing formation are used as conduits forcarrying a heated fluid into the formation. Heat from this fluid isemployed to reduce the viscosity of at least some oil in the upper andtransitionzones. The heated oil is then driven to a producing well, oris then entrained in the fluid passing through the water zone and thetransition zone and carried to a producing well. Heat from this thermaltreatment of the first treated formation is conducted through theadjoining impermeable barrier to an adjacent oil-bearing formationthereby reducing the viscosity and increasing the mobility of oil inthat layer. This conductively heated formation may then be treatedaccording to a hot water and/or steam flood recovery process.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a diagrammatic view partiallyin cross-section of alayered, subterranean, viscous oilcontainingreservoir traversed by two wells suitably equipped for the practice ofthis invention.

FIG. 2 is a vertical sectional view of the reservoir at a time after ithas been treated according to the teachings of this invention.

DESCRIPTION OF A PREFERRED EMBODIMENT Referring to FIg. 1, we see asubsurface oil reservoir 10, which encompasses a number of porous,permeable viscous oil-containing formations or intervals 11-14 separatedby a number of impermeable layers or barriers 15-17 preferably less than50 feet thick. The permeable formations 11-14 may, for example, becomposed of sandstone while the impermeable layers 15-17 may be composedof shale or other impervious rock capable of conducting heat. Thelowermost sand formation 14 has an upper zone 18 predominately saturatedwith a viscous crude oil and a lower zone 19 predominately saturatedwith water. The zones 18 and 19 meet at an oil-water contact 20.Adjacent the oilwater contact 20 may be a zone 20a of saturationtransition in which the oil saturation decreases and the watersaturation increases with increasing depth. Within at least some of thissaturation transition zone 20a, the relative permeability to both oiland water is great enough so that oil and water may be expected to flowsimultaneously through the zone 20a if the fluids have viscosities ofsimilar magnitude. However, because the viscosity of the viscous oil inthe formation 14 is much greater at the naturally occuring reservoirtemperature than the viscosity of the water in the formation 14 at thattemperature, the oil in the saturation transition zone 20a issubstantially immobile when at that temperature whereas at least aportion of the water in this zone 20a is mobile.

To recover oil from the reservoir 10 according to the method of thisinvention, at least a first well 21 and a second well 22 are extendedinto the oil reservoir 10. Each of the wells 21 and 22 at leastpartially transverses the predominately water-saturated lower zone 19 ofpermeable formation 14. The wells 21 and 22 may be completed in aconventional manner, as for example, by extending tubular casings 23 and24 into well bores 25 and 26 of the wells 21 and 22 and fixing thecasings 23 and 24 in place with cement 27 and 28. The wells 21 and 22may then be opened into fluid communication with preferably at least aportion of each of the zones 18, 19 and 20a of the formation 14 by anymethod in the art such as perforating the casings 23 and 24 and thesurrounding cement 27 and 28 with perforations such as those indicatedby numerals 29 and 30. The intervals of the well bores 25 and 26 whichare provided with perforations 29 and 30 preferably extend at least 2feet above and below the oil-water contact 20, the location of which maybe determined by methods well known in the art, such as electriclogging, as may the respective locations of the zones 18, 19 and 200.

A flow path for injecting fluid into formation 14 through the first well21 may be provided as by extending a string of tubing 31 into the well21 to location in the borehole 25 adjacent the formation 14. A packer 32may be set in the annulus between the tubing 31 and the casing 23 abovethe perforations 29 which open the borehole 25 into fluid communicationwith the formation 14 to pack off the perforate portion of the borehole25. The second well 22 may be equipped in any conventional manner, aswith a string of tubing 39 and a pump means 40 driven by a rod means 41,to produce fluid which flows into that well 22 from the formation 14through the perforations 30.

The formation 14 may first be preheated by injecting hot water orlow-grade steam (about percent quality) down the first well 21, throughthe perforations 29, and into the formation 14, and producing fluid fromthe formation 14 through the second well 22. Because the water in thepredominately water-saturated lower zone 19 and the transition zone 20ahas a greater mobility than the viscous oil in the predominatelyoil-saturated zone 18, the water in the zones 19 and 20a will move tothe second well 22 relatively rapidly compared to the rate of fluidmovement in zone 18.

As the water moves toward the well it is at the same time replaced byhot injected fluid which moves through the zones 19 and 20a heating oilin the zone 20a and conductively preheating oil in the oil-saturatedzone 18. As the oil in the transition zone 20a adjacent the oil-watercontact 20 is heated, it becomes more mobile. At least some of theheated mobile oil in the transition zone 20a is entrained in the movingstream of hot injected fluid and is moved along therewith to theproducing well 22 where the oil is produced along with the water inwhich it is entrained. Thus, oil production from the saturationtransition zone 20a may begin dur ing the preheat period before asubstantial portion of the oil in the upper zone 18 is significantlymore mobile than it is at the naturally occurring reservoir temperature.

Preferably, after hot water injection into the formation 14 iscommenced, the producing well 22 is at least from time to time checkedas by measuring the temperature of the produced fluids to determinewhether or not heat communication is established between the injectionwell 21 and the producing well 22. The injectivity of fluid into well 21may also be measured for indication of communication. Preferably afterheat communication between the injection well 21 and producing well 22is established, the injection of hot water or low grade steam is stoppedand relatively high grade steam (preferably of at least 75 percentquality) is then injected into the formation 14 through the establishedpaths of communication in zones 19 and 20a and through the predominatelyoil-saturated zone 18 making the oil in this zone 18 even more mobile.

According to a second embodiment of this invention, such relatively highgrade steam may be injected down the first well 21 and into theformation 14 as a preheating fluid without first injecting hot water orlow grade steam into the formation 10. This fluid initially moves intothe water-saturated lower zone 19 and the transition zone 20a asdescribed above to preheat oil in the upper zone 18, Preheating withthis higher quality steam has the advantage of transfering more heat(the heat of vaporization of the steam) to the formation 14 per unitmass of fluid injected than hot water preheating. However, because steamis less dense than water, the contribution to bottom hole pressure inthe well 21 of the column of fluid in the tubing 31 is less when steamis used as the preheat fluid. Therefore, for a given injection pressureat the top of the tubing string 31, the mass flow rate of injectedpreheat fluid may be lower if the preheat fluid is steam instead of hotwater.

Whether the preheat fluid is hot water or steam, after the preheatperiod, the lower zone 19 of the formation 14 may be at least partiallyresaturated with heated, more mobile oil by continuing to inject steaminto the formation 14. This steam forms a front 33 (FIG. 2) which mayrise as it advances through the formation 14 toward the producing well22 forming a predominately steam saturated zone 34 suchas thatillustrated. At the front 33, the advancing steam heats oil and drivesit toward the producing well 22. Simultaneously, at least some of heatedoil from above the steam zone 34 may drain down into the steam zone 34where it may be entrained in and carried with the steam advancing towardthe producing well 22. At least some heated oil may drain from the steamzone 34 into the lower zone 19 resaturating this zone with oil. As oilsaturation in the zone 19 increases, oil may be entrained in and carriedwith the injected fluid moving through this zone to the well 22.

Because the steam, at least initially, moves into the formation 14 morerapidly through the predominately water-saturated lower zone 19 thanthrough the viscous oil-saturated upper zone 18, it is to be expectedthat the steam front 33 first breaks through into the producing well 22from the lower zone-l9. The occurrence of steam breakthrough may bedetermined in a wellknown manner such as by observing the temperature ofthe produced fluids. When steam breakthrough occurs into the producingwell 22 the steam influx into the well may be shut off at least in partby plugging back the lower portions of the perforated interval of theborehole 26 of the well 22 by any method known in the art such as, forexample, filling the borehole with a gravel 35 and covering the gravelwith a cement cap 36. From time to time, as the steam zone 34 risesthrough the formation 14 and steam again breaks through into the well 22more of the perforated interval of the producing well 22 may be pluggedback.

During the period of hot fluid injection into the first heatedproductive formation 14, heat may be transferred by conduction throughthe adjoining shale barrier 17 and into the adjacent viscous oil-bearingformation 13. This heat reduces the viscosity of the oil in formation 13thus increasing the mobility and injectivity of that interval.Preferably, after it has been determined (as by temperature surveys,mathematical analysis or other methods well known in the art) that theinjectivity of the formation 13 has been increased to a selecteddesirable value, the wells 21 and 22 may be opened into fluidcommunication with the formation 13 as by perforating the casings 23 and24 with perforations 37 and 38 (if this has not been previously done). Aseparate confined path for fluid flow into the formation 13 may beprovided in the injection well 21 as by installing previously referredto packer 32 in the annulus between the tubing 31 and the casing 23 at aloca tion in the well bore 25 which is between the perforations 37 and29 which open into formations 13 and 14, respectively. This provides forthe simultaneous injection of separate columns of heated fluid down thetubing 31 and into formation 14 and down the annulus between the tubing31 and the casing 23 and into the formation 13.

Preheated viscous oil may then be driven from formation 13 by injectinghot water and/or steam into this formation through the first well 21while producing fluid from the formation through the producing well 22.As hot fluid moves through formation 13, heat is lost by conductionthrough shale barrier 16 to adjacent formation 12. This heat reduces theviscosity of the oil in formation 12 thus increasing the injectivity ofthat formation and facilitating the initiation of a thermal floodtherein. Thus, the reservoir may be produced by a stair-step'methodwhich begins by injecting heat into the predominately water-saturatedzone 18 and the transition zone a of formation 14 and leads to thethermal flooding of adjacent formations l3, l2 and 11, respectively. Itshould be understood that while in the embodiment of the inventionheretofore discussed only formations above the first treated formation14 were treated, it is within the concept of this invention to pre heatand subsequently thermally flood adjacent formations below the firsttreated formation 14.

In summary, the present invention provides a process for the recovery ofoil from a subsurface oil-bearing reservoir which includes at least afirst and a second productive formation which are adjacent but separatedby a substantially impermeable barrier through which heat may beconducted and wherein the first productive formation has three zones anupper zone predominately saturated with a viscous relatively immobileoil, a lower zone predominately saturated with water and a saturationtransition zone wherein oil saturation decreases and water saturationincreases with increasing depth. An embodiment of the method comprisesthe steps of: extending at least a first well and a second well to eachat least partially traverse the first and second productive formations;opening the wells into fluid communication with the upper zone, thelower zone, and the transition zone of the first productive formation;preheating the oil in the upper zone of the first productive formationby injecting hot water into the first productive formation through thefirst well whereby the hot water moves into the transition zone and intothe predominately water-saturated lower zone establishing a path of heatcommunication and carrying heat into the first productive formationwhich heats oil in the transition zone and which conductively heats atleast some of the oil in the upper zone; entraining in the hot watermoving through the transition zone of the first productive formation atleast a portion of the heated oil in the transition zone, and producingsaid entrained oil through said second well along with said water.

steam simultaneously into the upper, lower, and transition zones of thefirst productive formation through a single flow path in the first wellwhile producing fluid substantially simultaneously from the upper,lower, and transition zones of the first formation through a single flowpath in the second well.

Thereafter, one may open the first well and the second well into fluidcommunication with the adjacent second productive formation, provide aseparate confined path for fluid flow into the adjacent secondproductive formation through the first well, and drive oil from theadjacent second productive formation by injecting a heated fluid intothe second productive formation through the separate confined path forfluid flow in the first well while continuing to inject steam into thefirst productive formation. This oil may be produced from the secondproductive formation through the second well. As fluid is produced, onemay determine when steam breakthrough into the second well from thefirst productive formation occurs. After the steam breakthrough occurs,the steam influx into the second well may be at least partially shut offby plugging back at least some of that portion of the second well whichis open into fluid communication with the first productive formation.

We claim as our invention:

1. In a process for the recovery of oil from a subsurface oil-bearingreservoir which includes at least two productive formations, a firstproductive formation having three zones an upper zone predominatelysaturated with a viscous relatively immobile oil, a lower zonepredominately saturated with water, and a saturation transition zonewhere in oil saturation decreases and water saturation increases withincreasing depth and an adjacent second productive formation separatedtherefrom by a substantially impermeable barrier through which heat maybe conducted, the method comprising the steps of:

extending at least a first well and a second well to each at leastpartially traverse said first and second productive formations;determining the location in said first formation of said first zone,second zone and transition zone;

opening said first well and said second well into fluid communicationwith the upper zone, the lower zone, and the transition zone of saidfirst productive formation;

preheating at least some of the oil in the upper zone and heating atleast some of the oil in said transition zone of said first productiveformation by injecting a hot fluid into said first productive intervalthrough said first well whereby said hot fluid moves into saidtransition zone and said predominately water-saturated lower zonecarrying heat into said first productive formation which conductivelypreheats said upper zone;

entraining in hot fluid moving through said transition zone of saidfirst productive formation at least a portion of said preheated oil inthe transition zone;

producing said entrained oil through said second well along with saidfluid;

after said preheat step, driving oil to said second well andconductively preheating said adjacent second productive formation bycontinuously injecting steam simultaneouslyinto the upper, lower, andtransition zones of said first productive formation through a singleflow path in said first well; and

producing fluid from said upper, lower, and transition zones of saidfirst formation through a single flow path in said second well;

whereby said steam injected into said transition zone and said lowerzone moves toward said second well and rises' carrying heat into saidfirst productive formation further heating the oil in said upper zoneand entraining at least some heated oil draining from said upper zoneinto said transition zone and said lower zone, whereby said steaminjected into said upper zone further heats the preheated oil in saidupper zone and drives said oil in said upper zone to the second well,and whereby the adjacent second productive formation is preheated byheat conducted from said first productive formation through saidimpermeable barrier;

opening said first well and said second well into fluid communicationwith said adjacent second productive formation;

providing a separate confined path for fluid flow into said adjacentsecond productive formation through said first well;

driving oil from said adjacent second productive formation by injectinga heated fluid into said second productive formation through saidseparate confined path for fluid flow in said first well whilecontinuing to inject steam into said first productive formation;

producing oil from said second productive formation through said secondwell;

determining when steam breakthrough into said second well from saidfirst productive formation occurs; and

when said steam breakthrough occurs, plugging back at least some of thatportion of said second well open into fluid communication with saidfirst productive formation to shut off at least some of said steambreakthrough.

2. The method of claim 1 wherein said fluid is hot water.

1. In a process for the recovery of oil from a subsurface oilbearingreservoir which includes at least two productive formations, a firstproductive formation having three zones - an upper zone predominatelysaturated with a viscous relatively immobile oil, a lower zonepredominately saturated with water, and a saturation transition zonewhere in oil saturation decreases and water saturation increases withincreasing depth and an adjacent second productive formation separatedtherefrom by a substantially impermeable barrier through which heat maybe conducted, the method comprising the steps of: extending at least afirst well and a second well to each at least partially traverse saidfirst and second productive formations; determining the location in saidfirst formation of said first zone, second zone and transition zone;opening said first well and said second well into fluid communicationwith the upper zone, the lower zone, and the transition zone of saidfirst productive formation; preheating at least some of the oil in theupper zone and heating at least some of the oil in said transition zoneof said first productive formation by injecting a hot fluid into saidfirst productive interval through said first well whereby said hot fluidmoves into said transition zone and said predominately water-saturatedlower zone carrying heat into said first productive formation whichconductively preheats said upper zone; entraining in hot fluid movingthrough said transition zone of said first productive formation at leasta portion of said preheated oil in the transition zone; producing saidentrained oil through said second well along with said fluid; after saidpreheat step, driving oil to said second well and conductivelypreheating said adjacent second productive formation by continuouslyinjecting steam simultaneously into the upper, lower, and transitionzones of said first productive formation through a single flow path insaid first well; and producing fluid from said upper, lower, andtransition zones of said first formation through a single flow path insaid second well; whereby said steam injected into said transition zoneand said lower zone moves toward said second well and rises carryingheat into said first productive formation further heating the oil insaid upper zone and entraining at least some heated oil draining fromsaid upper zone into said transition zone and said lower zone, wherebysaid steam injected into said upper zone further heats the preheated oilin said upper zone and drives said oil in said upper zone to the secondwell, and whereby the adjacent second productive formation is preheatedby heat conducted from said first productive formation through saidimpermeable barrier; opening said first well and said second well intofluid communication with said adjacent second productive formation;providing a separate confined path for fluid flow into said adjacentsecond productive formation through said first well; driving oil fromsaid adjacent second productive formation by injecting a heated fluidinto said second productive formation through said separate confinedpath for fluid flow in said first well while continuing to inject steaminto said first productive formation; producing oil from said secondproductive formation through said second well; determining when steambreakthrough into said second well from said first productive formationoccurs; and when said steam breakthrough occurs, plugging back at leastsome of that portion of said second well open into fluid communicationwith said first productive formation to shut off at least some of saidsteam breakthrough.
 2. The method of claim 1 wherein said fluid is hotwater.